Invasive herbivores are often found co-occurring at high densities with invasive plants in eutrophic aquatic ecosystems but this may reflect common responses of each to nutrient enrichment, common dispersal pathways, or an effect of nutrient enrichment on their interactions. We investigated whether nutrient enrichment had a positive effect on the growth of invasive aquatic plants and herbivores using common exotic species. Forty-five 19-liter containers were assigned two treatments in a factorial design: plant identity [Eichhornia crassipes (water hyacinth), Alternanthera philoxeroides (alligator weed), or lettuce] and nutrient additions [zero, low, and high]. Each container received fifteen juvenile invasive Pomacea insularum (island apple snails).

Results/Conclusions

We collected data on plant and island apple snail growth after 9 weeks. Island apple snails had higher growth rates with lettuce compared to either exotic plant treatment, which indicates that these plants are a well defended food source. Island apple snails had higher growth rates at higher nutrient levels but there was a significant interaction between nutrients and plant identity. Invasive plant growth increased with nutrients. Nutrient input increased island apple snail growth in the invasive plant but not in the lettuce treatment where island apple snail performance was comparable and very high at all nutrient levels. Our results suggest that the high abundance of invasive herbivores and plants in high nutrient conditions is due to the positive effects of nutrients on plants and the indirect positive effects of nutrient addition on island apple snails.